Skip all separators (see is_separator of class CHARACTER) and make the first non-separator
available in last_character.

This non-separator character is pushed back into the stream (see
unread_character) to be available one more time (the next read_character will consider this
non-separator). When end_of_input occurs, this process is automatically stopped.

% Make the result available in last_string
common buffer. The end of line character (usually '%N') is not added in the last_string buffer.
Read tutorial or io cluster documentation to learn the read_line usage pattern.

The result is available in the last_string
common buffer. Heading separators are automatically skipped. Trailing separators are not skipped
(last_character is left on the first one). If end_of_input is encountered, Result can be the
empty string.